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VNODE(9) FreeBSD Kernel Developer's Manual VNODE(9)
NAMEvnode -- internal representation of a file or directory
SYNOPSIS#include <sys/param.h>
#include <sys/vnode.h>
DESCRIPTION
The vnode is the focus of all file activity in UNIX. A vnode is
described by structvnode. There is a unique vnode allocated for each
active file, each current directory, each mounted-on file, text file, and
the root.
Each vnode has three reference counts, v_usecount, v_holdcnt and
v_writecount. The first is the number of clients within the kernel which
are using this vnode. This count is maintained by vref(9), vrele(9) and
vput(9). The second is the number of clients within the kernel who veto
the recycling of this vnode. This count is maintained by vhold(9) and
vdrop(9). When both the v_usecount and the v_holdcnt of a vnode reaches
zero then the vnode will be put on the freelist and may be reused for
another file, possibly in another file system. The transition to and
from the freelist is handled by getnewvnode(9), vfree(9) and vbusy(9).
The third is a count of the number of clients which are writing into the
file. It is maintained by the open(2) and close(2) system calls.
Any call which returns a vnode (e.g. vget(9), VOP_LOOKUP(9) etc.) will
increase the v_usecount of the vnode by one. When the caller is finished
with the vnode, it should release this reference by calling vrele(9) (or
vput(9) if the vnode is locked).
Other commonly used members of the vnode structure are v_id which is used
to maintain consistency in the name cache, v_mount which points at the
file system which owns the vnode, v_type which contains the type of
object the vnode represents and v_data which is used by file systems to
store file system specific data with the vnode. The v_op field is used
by the VOP_* macros to call functions in the file system which implement
the vnode's functionality.
VNODE TYPES
VNON No type.
VREG A regular file; may be with or without VM object backing. If you
want to make sure this get a backing object, call
vfs_object_create(9).
VDIR A directory.
VBLK A block device; may be with or without VM object backing. If you
want to make sure this get a backing object, call
vfs_object_create(9).
VCHR A character device.
VLNK A symbolic link.
VSOCK A socket. Advisory locking will not work on this.
VFIFO A FIFO (named pipe). Advisory locking will not work on this.
VBAD An old style bad sector map
IMPLEMENTATION NOTES
VFIFO uses the "struct fileops" from /sys/kern/sys_pipe.c. VSOCK uses
the "struct fileops" from /sys/kern/sys_socket.c. Everything else uses
the one from /sys/kern/vfs_vnops.c.
The VFIFO/VSOCK code, which is why "struct fileops" is used at all, is an
artifact of an incomplete integration of the VFS code into the kernel.
Calls to malloc(9) or free(9) when holding a vnode interlock, will cause
a LOR (Lock Order Reversal) due to the intertwining of VM Objects and
Vnodes.
SEE ALSOmalloc(9), VFS(9)AUTHORS
This manual page was written by Doug Rabson.
FreeBSD 10.1 May 20, 2003 FreeBSD 10.1